Method and apparatus for continuously producing a monomolecular film at a liquid gas interface in a trough by means of a rotating barrier element

ABSTRACT

The invention concerns a method for continuously producing a monomolecular film at a liquid-gas interface in a trough, by means of a barrier element rotating horizontally around a vertical shaft, which barrier moves the film from a film formation compartment through a connection zone to a deposition compartment to be deposited onto a substrate. On the opposite side of the barrier with respect to the rotational direction of the barrier element, more film forming substance is added to the liquid surface for forming a new film, which in turn is transferred by a rotating barrier element at a suitable surface pressure to elastically join the first mentioned film as an extension to the same. The invention also concerns a trough apparatus for continuously producing films.

This is a national-stage application of PCT/FI90/00298, filed Dec. 14,1990.

The invention concerns a method for continuously producing amonomolecular film on a liquid surface for the deposition thereof onto asubstrate. The invention also concerns an apparatus for continuouslyproducing a monomolecular film.

BACKGROUND OF THE INVENTION

A number of compounds, one end of which is water-soluble and the otherend fat-soluble, form onto the interface of a liquid phase and a gasphase an insoluble film having the thickness of one molecule layer. Inthis film the said amphiphilic compounds are oriented so that thewater-soluble end of the compound is in the liquid phase and thefat-soluble end in the gas phase. Irwin Langmuir's original publication(Langmuir, I. (1917) J. Am.Chem. Soc. 39, 1848) describes the principlesaccording to which the orientation of compounds into monomolecularlayers is accomplished. Different methods for producing and studyingsurface films have been widely dealt with in a monograph by GeorgeGaines (Gaines, G. L. , Jr (1966) Insoluble Monolayers at Liquid-GasInterfaces, interscience Publishers, John Wiley and Sons Inc., New York1966).

To make monomolecular surface films, the chosen substance, dissolved ina volatile solvent, such as e.g. chloroform or hexane, is spread onto aliquid surface in a trough. The available surface area of the liquid-gasinterface is limited by a barrier resting on top of the trough, incontact with the liquid surface, and by means of which the total area ofthe monomolecular surface film is either decreased or increased. Thus itis possible to regulate the surface tension of the film which isinversely proportional to the surface pressure of the film. The degreeof packing, that is the surface density of the molecules of the film,can be measured by using the so called compression isotherm, bydetermining the surface pressure as a function of the surface area, inorder to obtain the surface area/molecule vs. surface pressure. Byarranging for a feedback coupling between the surface pressure sensor(usually a Wilhelmy-plate, dunoyu-ring or a Langmuir-barrier) and themotor controlling the movements of the barrier, it is possible to createconditions where the surface pressure of the film, that is the packingdensity of the molecules, is maintained constant. This is necessary whenthe film on the surface of the liquid, typically of water, is to betransferred onto a solid substrate to be coated that can be plastic,glass, a silicon wafer, metal or alike. As a result of one transfer, onelayer is deposited onto the substrate. This kind of a monomolecular filmtransferred onto a solid substrate is called a Langmuir-Blodgett film.By repeating the transfer, it is possible to produce multilayerLB-films.

LB-films attract nowadays great interest because they open new technicalprospects i.a in the semiconductor industry as resists, in datacommunication as photoconductors, in laser technique in the SHGformation as well as in the sensor technique. A problem has been that sofar there has not been commercially available a so-called continuouslyoperating LB-method and apparatus.

The U.S. Pat. No. 4,093,757 describes an apparatus and a method fordepositing a monomolecular layer of an amphiphilic compound onto asubstrate. The apparatus consists of two compartments and thecompartments are separated by a horizontally rotatable cylinder partlyimmersed in the liquid and feeding molecules of the amphiphilic compoundfrom one compartment to the other. The continuous deposition andproduction of the film is performed by adding dropwise, at apredetermined rate, a film compound, dissolved in an organic solvent,into one compartment and continuously transferring the said compound tothe other compartment by rotating the cylinder.

The U.S. Pat. Nos. 4,722,856 and 4,783,348 describe an apparatus ofanother type for continuously producing films, wherein no movingmechanical components are used in contact with the film, but the film iscompressed and transported from one region of the trough to anotherusing a continuous medium flow, either by blowing gas above the liquidsurface tangentially to the surface or by draining the surface from anupper level to a lower level and continuously adding the film compoundto the upper compartment.

SUMMARY OF THE INVENTION

Now a method and apparatus have been invented that permit the continuousproduction of films by simple means, where the film is controllablydeposited onto a substrate and where efficient deposition rates can beattained if desired, e.g. a rate of 30 M² /h. This is attainable by themethod in accordance with the present invention for continuouslyproducing a monomolecular film on a liquid surface in a trough for thedeposition thereof onto a substrate, the surface area of the film beingdefined by a barrier element and the side walls of the trough, whichmethod is characterized in that the barrier element, pivoted at one endto perform a horizontally rotating movement along the surface in a filmformation compartment of the trough, gradually transfers the film, at asurface pressure suitable for deposition, to a deposition compartment ofthe trough connected to the film formation compartment over a connectionzone, for the deposition of the film onto the substrate, wherebyadditional film forming substance is introduced to the surface of theliquid in the film formation compartment, on the opposite side of thesaid barrier element with respect to its rotational direction, for theformation of a new film, which new film, when the radially outer end ofthe barrier element, due to the further rotational movement thereof, hasreached the connection zone and surface contact has been establishedbetween the film formation and deposition compartments, at the requiredsurface pressure is transferred by a barrier element to join the saidfirst mentioned film already transferred to the deposition compartment.

According to the most simple embodiment, only one barrier element isused, one end of which is rotatably mounted. To compensate for filmdeposited onto the substrate in the deposition compartment, the movingbarrier gradually pushes the film from the film formation compartment ofthe trough to the said separate deposition compartment. When thebarrier, or specifically its radially outer end section during itsrotating movement, has reached the connection zone, the film will bealmost completely transferred to the deposition compartment and at thispoint it will still be separated from the main body of the filmformation compartment by the barrier. However, there now follows a stagewhere the surface in the connection zone will join that of the filmformation compartment, due to the further barrier movement. Thiscorresponds substantially to the stage where the barrier is in anangular position where its outer end has moved away from the top of thewall of the film formation compartment into the connection zone. Beforethis stage is reached, however, enough film substance for forming a newfilm layer has been added, in accordance with the invention, onto thesurface of the liquid in the film formation compartment, on the oppositeside of the barrier with respect to the rotational direction. This newfilm will be subsequently transferred to form an extension to the saidfirst film.

In the embodiment using only one barrier element, the said barrierelement will, at the point just prior to the joining of the films, dueto the suitable design of the film formation compartment, extend bothacross the width of the trough in the connection zone and also intersectthe film formation compartment, the barrier element thus beingsimultaneously in contact with the first mentioned film in thedeposition zone and the newly formed film in the film formationcompartment. When surface contact at the connection zone is established,as stated above, the one and the same barrier elastically moves the newfilm previously made in the film formation compartment to join the filmalready in the deposition compartment. The further movement of thebarrier then pushes this joined film gradually to the deposition stationas it is used for deposition onto the substrate, all the timemaintaining the required surface pressure in the film. During each fullcircle of the barrier, one new film is formed to be transferred into thedeposition compartment.

It is to be noted that the geometry of the trough and the barrierelement has to be such that at the stage of establishing surfacecontact, as described above, when the barrier rotates over an angle s,the change in surface area (dA₁ /ds) in the connection or depositionzone, has to be equal to or smaller than the change of surface area (dA₂/ds) in the film formation compartment for compensation to take place.However, as the deposition also uses film, the rate of rotation of thebarrier at this stage has to be carefully controlled in order tomaintain constant surface pressure.

According to one alternative embodiment there are two barriers operatedsynchronically, whereby the first barrier moves the film to thedeposition compartment as the film is used, maintaining constant surfacepressure, whereas the second barrier together with the first mentionedbarrier together with the walls of the trough define an area of the filmformation compartment wherein film forming substance is added to form anew film to be transferred by the said second barrier to the depositioncompartment after the first mentioned barrier or rather its outer endhas reached the connection zone and surface contact at this point hasbeen established. According to one variation of this embodiment thereare three barriers to be operated synchronically, whereby, at each stageof the film formation process, always one barrier is substantiallyimmobile, forming adjacent the connection zone a radially extendingseparation wall preventing film from slipping from the connection zoneback into the film formation compartment, whereas the two other barriersfunction as described above. When a barrier has passed the connectionzone and hits the immobile barrier, the latter starts its rotatingmovement whereas the first mentioned remains immobile during thefollowing film formation cycle. Naturally the rates of the barriers, andthe change in surface area of the films, have to be adjusted so that,when surface contact is established at the connection zone, propercompensation takes place, as stated above, and the films join smoothly.

The apparatus for continuously producing a monomolecular film at aliquid surface according to the invention comprises a trough forreceiving the liquid and at least one horizontally rotatable barrierelement mounted at one end on a vertical shaft or shafts in contact withthe liquid surface and resting on the rims of the trough, which barriertogether with the walls of the trough defines the surface area of thefilm to be formed, and the apparatus is characterized in that the troughincludes a film formation compartment comprising the said barrierelement(s), for transferring the film from the film formationcompartment into a deposition compartment, which joins the filmformation compartment over a connection zone, for receiving the film tobe deposited onto a substrate.

In its widest sense the apparatus of the invention comprises atwo-compartment trough, namely a primary film formation compartmentwhere the barrier element or the barrier elements are mounted at one endand where they perform their rotating movement, and a depositioncompartment. According to one advantageous embodiment the shape of thefilm formation compartment is substantially annular, the depositioncompartment joining the film formation compartment substantiallytangentially, the connection zone then being the area in the depositionzone lying immediately outside and adjacent the imaginary periphery ofthe annular film formation compartment. The film formation compartmentmay also have the shape of an open ring, or rather of a spiral or ahook, the outer extension of the spiral/hook connecting to thedeposition compartment. The inner end part of the spiral/hook will thenbe physically separated from the deposition chamber. The barrier elementor elements are mounted from one end in the middle of the ring orspiral. According to one variation of this embodiment, the filmformation compartment is substantially shaped like a hooked U, wherebythe mounting point of the barrier element or elements during theirsweeping circular movement over the leg and the bottom parts of the U,moves back and forth substantially along a rectilinear path between thelegs of the U. In this arrangement, the deposition compartment joins toone leg of the U as an extension to the latter. The shape and size ofthe deposition region are not critical in regard to the invention, aslong as it is large enough for allowing deposition of the film onto thesubstrate and for measuring the surface pressure. The above describedspiral or hookshaped embodiments are especially favourable for use withone barrier element only, although it is quite conceivable to use two ormore barriers as well. When using the embodiment with only one barrier,the barrier must at least at one stage of the process simultaneouslyintersect both the inner end of the spiral or hook, and the connectionzone, as stated above, in order for the barrier to simultaneously be incontact with the newly formed film as well as the old film and toelastically join these two. The shape of the barrier element may vary,especially the part of the barrier which coincides with the connectionzone can be shaped in such a way that surface contact is effected assmoothly as possible.

According to one alternative embodiment the film formation compartmentis substantially circular, the barriers being rotatably and coaxiallymounted on vertical axes in the center of the circle and extendingradially from the center over the walls of the trough. The depositioncompartment is advantageously connected to the film formationcompartment substantially tangentially, whereby the connection zone ofthe said compartment will be the area on the outside of and immediatelyadjacent the imaginary periphery of the film formation compartment. Itis, however, preferable in this embodiment to use two independentlymounted, but synchronously operated barriers, so that always one of thebarriers is transferring a film for deposition, while a new film isbeing formed in the region defined by the said two barriers,subsequently to be joined as an extension to the previously formed filmand then to be moved to the deposition compartment to enable continuousdeposition onto the substrate. According to one alternative constructionof this embodiment, there are three barriers, whereby always one barrierforms, in close vicinity to the connection zone, an immobile radiallyextending separating wall. This construction facilitates the completetransfer of the film in a desired manner to the deposition compartmentpreventing any film substance from escaping back into the film formationcompartment at the connection zone. The barriers are in this casepreferably constructed so that they diverge symmetrically from thecenter outwards so that a barrier can rest tightly against anotherbarrier acting as a separation wall, thus promoting the completetransfer of the film to the deposition compartment and making sure thatthe apparatus remains clean.

Thus the invention makes it possible to make and to transfer atwo-dimensional film to be continuously deposited onto a substrate witha device that operates like a pump or bellows. As mentioned above, thesurface pressure can be measured e.g., as already known, by using asurface pressure sensor in the deposition compartment of the trough,which sensor is coupled back, e.g. through a computer to a motorcontrolling the movements of the barrier or barriers, whereby it ispossible, with several barriers, to regulate their mutual speedssuitably, taking also into account the rate at which the film formingsubstance is added to the trough. When using a substantially U-shaped,longitudinal trough, the pivotal point of the barrier can be joined to aconveyor or belt that moves around two wheels located between the legsof the U. Surface pressure measurement and mutual speed regulation ofthe barriers on the basis of the measured value, as well as the mannerof mounting the barriers are known techniques and are not included inthis invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail in the following, withreference to the attached drawing, wherein

FIG. 1a shows schematically a top view of a first embodiment of theapparatus according to the invention, and FIG. 1b shows a schematicaland perspective view thereof,

FIG. 2 shows schematically a top view of a second embodiment of theinvention, and

FIGS. 3a, 3b, 3c, and 3d show schematically a third embodiment of anapparatus according to the invention, comprising three barriers, whereinFIGS. 3a-3d illustrate different positions of the barriers at differentstages during one film production cycle.

DETAILED DESCRIPTION

FIG. 1a shows schematically a top view of the apparatus 1 of thisinvention including a trough to--2 and FIG. 1b a perspective viewthereof the trough is substantially shaped like a spiral or a hook andsuitably produced by machining, e.g. by hollowing a body of a suitablematerial to form a trough which as a whole is designated by referencenumeral 2. The top rim of the trough walls 3 forms the top surface ofthe trough. The trough is filled up with a liquid used as subphase sothat the interface of the liquid supporting the film is just at the samelevel with the top surface of the trough. The internal surfaces, as wellas the top surface of the trough are preferably covered by a hydrophobicmaterial, e.g. by tetrafluoropolyethylene (Teflon). The trough comprisestwo compartments, the so called film formation compartment 4, mainlycorresponding the annular part of the spiral shaped trough, the closedinner end of which being marked with the reference 4a, and thedeposition compartment 5, only part of which is shown in the figure,namely mainly the connection zone 6 from the film formation compartmentto the deposition compartment. The trough further comprises a barrier 7that is rotatably mounted on a vertical shaft 8 approximately in themiddle of the trough. In FIG. 1a the barrier is shown in two differentstages a and b of the film formation process, the one position a of thebarrier being marked with a dashed line 7'. The length of the barrier 7is such that it extends at its every angular position across the liquidsurface in the film formation compartment 4. Together with the walls ofthe trough it defines an area of the liquid surface for the film. Theapparatus functions as follows. Let us assume that the barrier is atpoint a. The film has already been formed at the liquid-gas interface onthat side of the barrier which is in the direction of rotation and facesthe connection zone. The barrier moves at a given speed, dependent onthe deposition speed of the substrate, counterclockwise whilemaintaining the surface pressure at a value suitable for deposition ontothe substrate. Thus the barrier moves the film gradually to thedeposition compartment 5. At the same time enough film substance for a"new" film is added on the opposite side of the barrier. When thebarrier reaches position b, it extends simultaneously both across thefilm formation compartment, specifically across its end part 4a, andacross the connection zone 6 separated by the wall 4b from the end part4a. The barrier is at this stage at the same time in contact both withthe new film and the film formed during the previous cycle. The furthermovement of the barrier in the rotation direction makes the outer end ofthe barrier leave the top of the wall 3 and establishes surface contactbetween the connection zone 6 and the film formation compartment 4,whereby the new film joins the film already in the connection zone. Asmooth joining of the films is secured by regulating the amount of filmsubstance added, as well as the speed of the barrier. From now on thebarrier 7 continues to push the joined film in the compartment 4 to theconnection zone at the required surface pressure. In this way, by meansof the continuous speed-controlled rotational movement of the barrier onthe basis of the surface pressure measurement, continuous filmproduction and continuous depose-lion of the film onto the substrate canbe attained. The barrier is suitably made of a hydrophilic material,e.g. of Delrin plastic. Due to the material chosen for the trough thebarrier slides almost frictionlessly along the top surfaces of thetrough apparatus.

According to an alternative embodiment operating substantially on thesame principle, and schematically described in FIG. 2 with the samereference numbers as in FIG. 1, the trough is shaped like a longitudinalU, comprising a film formation compartment 4 and a depositioncompartment 5 and a connection zone 6 connecting these two. The mostessential difference between this and the embodiment according to FIG. 1is the path of the barrier 7 or its fastening point 8. In thisembodiment the barrier is fixed at one end 8 to a conveyor or belt 9which moves along a longitudinal path around two wheels 10 and 10'. Theconveyor 9 can be any, for this purpose suitable conveyor, e.g. a chainconveyor, whereby the wheels 10, 10' may accordingly be toothed wheelsor similar. While the pivotal point 8' is substantially moving back andforth between the two legs of the U, the barrier 7 makes a longitudinal,rotational movement sweeping over the film formation compartment 4 in amanner corresponding to the embodiment according to FIG. 1. Twoalternative positions of the barrier have been shown at point a (withbarrier 7' and pivotal point 8' shown in dashed lines) and point b,where the barrier is in the connection zone just before surface contactis established and the films on both sides of the barrier join. In thisposition the barrier intersects both the end part 4a of the filmformation compartment and the connection zone, as described above.

One further embodiment according to the invention is described in FIGS.3a-3d, Corresponding parts have been marked, where applicable, with thesame reference numbers as in FIGS. 1 and 2. The apparatus comprisesthree synchronously operated barriers 7a, 7b and 7c, the movement ofwhich is controlled by means of the surface pressure measured in thedeposition compartment 5. In this embodiment the trough is substantiallycircular and the barriers coaxially but independently mounted on avertical shaft at 8. The deposition compartment, or its one wall 11, isconnected over the connection zone 6 substantially tangentially to thefilm formation compartment. At the stage of the film formation processaccording to FIG. 3a, the barrier 7a is moving a film in its rotatingdirection to the deposition compartment. Before the barrier 7a hasreached the connection zone, film substance is added to the other sideof the barrier 7a, onto the liquid surface in the surface area definedby the barriers 7a and 7b. The barrier 7c is during this cycle immobileacting as a separation wall adjacent the connection zone 6. In thesituation according to the FIG. 3b, the barrier 7a has alreadyapproached the connection zone and just established surface contactbetween this and the film formation compartment, and the barrier 7b hascorrespondingly moved a little forward to maintain the required surfacepressure. The films on both sides of the barrier 7a are elasticallyjoined at the correct surface pressure. At the stage according to theFIG. 3c, the barrier 7a has passed over the connection zone to restagainst the barrier 7c. Thus any film material between the barriers 7aand 7c is forced to move to the connection zone. At the same time thebarrier 7b has moved forward and is in turn pushing the joined film viathe connection zone to the deposition compartment. During the next cyclethe barrier 7a functions as a separation wall, the barrier 7c in turnstarts a circular path and during this cycle film forming substance isadded to the surface region between the barriers 7b and 7c. Asillustrated by the figures, the situation according to the FIG. 3dcorresponds with the situation of the FIG. 3a, except that the barriershave changed places. In this embodiment, the barriers are preferablyslightly diverging from their pivotal points, thus forming sharp-angled,equilateral triangles that rest tightly against the side of anotherbarrier.

This embodiment can also be realized by an arrangement where there areonly two barriers and the immobile separation barrier is omitted. Inthis case, however, some film might slip from the connection zone backto the film formation compartment, whereby the liquid surface has to becleaned every now and then, e.g. by sucking possible unwanted materialsaway from its surface. In this embodiment it is advantageous that theradially outer end of the barrier is broad enough so that its endsurface, at least in some position of the barrier, intersects theconnection zone and momentarily cuts off surface contact between theconnection zone and the film formation compartment, the end surface thusacting as a separation wall. The film forming substance is added to theliquid surface e.g. by a computer controlled liquid feeder. The point ofthe applicator touches the liquid-gas interface during the application.The point of the applicator is moved away from above the trough by acomputer controlled electric motor, to leave room enough for the barriermovement.

Operation of the apparatus is controlled by a microprocessor connectedbetween the surface pressure sensor and the electric motor or motorsdriving the barrier system, which microprocessor also controls, also byan electric motor, the feeder of the film forming substance and itsapplicator part. To indicate the barrier position (surface area of thefilm) the barrier is advantageously feedback connected with themicroprocessor. The surface pressure is advantageously made to act onthe electric motor which drives the barrier which at the moment is inthe process of moving a film to the deposition compartment. The angularspeed of the barrier following the first mentioned barrier is determinedaccording to the speed (position) of the first mentioned barrier andpossibly according to the rate of addition of the film formingsubstance. Anyway, the rate of addition is preferably regulated to aconstant and optimal value for each system. If necessary, the system canalso be equipped with a suction apparatus to clean the surface fromremnants of film forming substances.

What is claimed is:
 1. Method for continuously producing a monomolecularfilm on a liquid surface in a trough for the deposition thereof onto asubstrate, a surface area of the film being defined by a barrier elementand side walls of the trough, the method comprising the steps of:pivoting the barrier element at one end to perform a horizontallyrotating movement along the surface in a film formation compartment ofthe trough, gradually transferring the film by means of the barrierelement, at a surface pressure suitable for deposition, to a depositioncompartment of the trough connected to the film formation compartmentover a connection zone, for the deposition of the film onto thesubstrate, and introducing additional film forming substance to thesurface of the liquid in the film formation compartment, on the oppositeside of the barrier element with respect to its rotational direction,for the formation of a new film, which new film, when the radially outerend of the barrier element, due to the further rotational movementthereof, has reached the connection zone and surface contact has beenestablished between the film formation and deposition compartments, atthe suitable surface pressure is transferred by the barrier element tojoin the first mentioned film already transferred to the depositioncompartment.
 2. Method of claim 1, wherein the pivoted end of thebarrier element moves along a substantially longitudinal path when thebarrier element performs its rotational movement.
 3. Method of claim 1or 2, wherein there is only one barrier element, which at least in oneposition during its rotational movement is in contact with the film bothin the film formation compartment and in the connection zone.
 4. Methodof any of the claims 1-2, wherein there are a plurality of barrierelements and mutual rotational speeds of the barrier elements areregulated on the basis of a surface pressure measured at the depositioncompartment.
 5. Apparatus for continuously producing a monomolecularfilm at a liquid surface, which apparatus comprises a trough havingwalls and a rim for receiving the liquid and at least one horizontallyrotatable barrier element mounted in contact with the liquid surface onthe rim of the trough, which barrier element together with the walls ofthe trough defines a surface area of the film to be formed, and whereinthe trough includes a substantially annular film formation compartmentand a separate deposition compartment which extends substantiallytangentially from the film formation compartment over a connection zone,and wherein the at least one barrier element is positionable to contactboth a new film in the film formation compartment and the film formedduring a previous cycle in the connection zone to enable transfer of thefilm from the film formation compartment over the connection zone intothe deposition compartment to be deposited onto a substate.
 6. Apparatusof claim 5, wherein the film formation compartment is substantiallyU-shaped and one end of the barrier element is mounted at a point thatduring the rotational movement of the barrier element moves along asubstantially longitudinal path back and forth between the legs of theU.
 7. Apparatus of claim 5, wherein the film formation compartment issubstantially spiral-shaped and wherein the barrier element in at leastin one of its positions reaches both across the breadth of the filmformation compartment and across the connection zone.
 8. Apparatus ofclaim 5, wherein the film formation compartment is substantiallycircular and there are at least two barrier elements independently andcoaxially mounted, whereby at each stage of the film formation process afirst barrier element defines together with the walls of the depositioncompartment and the connection zone, a surface area of the film to bedeposited onto the substrate, and a second barrier element together withthe first barrier element in the film formation compartment define asurface area of a film to be subsequently deposited onto the substrate.9. Apparatus of claim 8, wherein there are three barrier elements, thethird barrier element being at every stage of the film formation processimmobile.
 10. Apparatus of claim 5, wherein the film formationcompartment is substantially circular and at least a portion of thebarrier element acts as a separation wall between the film formation anddeposition compartments.